Sometimes when a chunk with chests is loaded the chests will turn invisible then when you open them all items inside are deleted from existence. (signs connected to the chest will also turn invisible and will lose all writing on them)
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can you be more spesific, is the items in the cheast being inviseble or dissapear or the cheast itself, or both? please update you description so it will be easier to understand. (I mean no harm, just want it to be a more helpfull report) If you could also describe how to recrate the issue, it would be very useful